When an image display apparatus displays on a display screen a video signal input from another device, the user viewing the display screen performs settings and adjustments to the display image such as the color of the display image displayed on the display screen, the position of the display image in the horizontal and vertical directions, contrast, brightness and the like in accordance with an OSD (On Screen Display) menu.
The OSD menu is displayed as an image superimposed on a predetermined area in the display image based on an input video signal. The user performs the aforementioned settings and adjustments to the display image by operating the image of the OSD menu in the display image with a button or knob or a touch panel or the like of the image display device main body.
When displaying an image of the OSD menu, the image display device performs a process of superimposing, that is, combining, the image of the OSD menu on a display image based on an input video signal for display as a composite image on the display screen.
Here, the resolution during display of the composite image in which the image of the OSD menu is combined is also determined based on the respective frequencies of the vertical synchronization signal and the horizontal synchronization signal of the display image based on the video signal. The frequency of each of the vertical synchronization signal and the horizontal synchronization signal is detected from the video signal supplied to the image display device.
For this reason, when the video signal is no longer supplied due to a sudden loss of the video signal supply source or the like, the respective frequencies of the vertical synchronization signal and the horizontal synchronization signal for displaying the display image based on the video signal cannot be obtained. Thereby, the image display device cannot determine the resolution of the display image based on the video signal, cannot generate the display image to be displayed on the display screen, and can no longer continue the process of displaying on the display screen the composite image in which the OSD menu is superimposed.
Therefore, even when the user is displaying the image of the OSD menu and setting and adjusting the display image of the display screen, when the supply of the video signal is lost, the image display device can no longer display the composite image, and so the display screen darkens to an all-black state. As a result, the OSD menu is not displayed either, and the usability in setting and adjusting the display image of the display screen is impaired.
In addition, when the respective frequencies of the vertical synchronization signal and the horizontal synchronization signal cannot be detected from the image of the image signal during startup of the image display device or when the format of the image signal has changed, since the display image cannot be normally displayed the image of the OSD menu is also not displayed.
In order to deal with a video signal in which the respective frequencies of the vertical synchronization signal and the horizontal synchronization signal described above cannot be detected, there is disclosed an image display device that requests a video signal for adjustment from the supply source of the video signal, detects the respective frequencies of the vertical synchronization signal and the horizontal synchronization signal from the video signal for adjustment, and displays the display image (see, for example, Patent Document 1).